1. Field of the Invention
This invention relates generally to touchscreen devices, and more particularly to capacitive touch systems.
2. Description of the Background Art
Currently, touchscreens are being used in embedded systems such as Smartphones, MP3 players, tablet computers, navigation systems, automatic teller machines (ATMs), and others. Traditionally, touchscreens were extremely expensive to manufacture and, therefore, impractical for most applications. However, more cost effective manufacturing processes have since been developed and touchscreen technology is rapidly gaining popularity in the electronics industry. Indeed, many conventional human-computer interface (HCI) input devices (e.g., keypads/keyboards, mechanical buttons, dials, etc.) are being replaced with touchscreens. Therefore, it is expected that the use of touchscreens in embedded applications will continue to increase for the foreseeable future.
A touchscreen system typically includes a transparent touch-sensor panel used in conjunction with an underlying graphical display device (e.g., liquid crystal display). The touch-sensor panel receives user inputs by detecting the location of a target object such as, for example, a finger, stylus, etc. The display device projects graphical output images directly through the touch-sensor panel such that human-computer interaction takes place at the touch surface of the touch-sensor panel.
One type of touch-sensor panel, the resistive touch panel, is used in many electronic devices today. A resistive touch panel is composed of two conductive layers, one of which deforms under pressure when it is touched by a target object. When the deformable conductive layer contacts the underlying conductive layer, a change in resistance between the layers is generated. A controller uses this change in resistance to determine the location of the touch.
Although resistive touch panels are still widely used, the overall design has several problems. For example, the deformable conductive layer has to be constructed from an extremely soft material in order to flex under low pressure. Consequently, the deformable layer can be punctured very easily, or damaged by abrasive or caustic cleaners. Moreover, the deformable layer can eventually fatigue over time and become “stretched” resulting in loss of touch sensitivity. As another example, resistive touch panels typically have poor optical quality because of the relatively low transparency of the materials from which most deformable layers are made. Another contributing factor to the poor optical quality is that the deformable layer tends to disperse light when deformed thereby causing the underlying display image to appear temporarily distorted.
The aforementioned problems associated with resistive touch panels are inherent to their fundamental design and operation. As a result, manufacturers and designers have been migrating away from resistive touch panels and toward the development of capacitive touch-sensor panels. The main advantage to capacitive touch-sensor panels is that they are not mechanically actuated. Instead, they locate target objects by sensing the presence of their electrical charge. Indeed, a target object need not necessarily make contact with the capacitive touch-sensor panel in order to be detected. This effectively eliminates the need for any flexible or moving parts. Accordingly, the touch surface of a capacitive touch-sensor panel is typically defined by a rigid transparent plate (i.e. glass), which has a much higher transparency and, therefore, optical quality than that of flexible touch surfaces. Furthermore, a glass touch surface does not deform and is, therefore, not susceptible to fatigue over time.
A capacitive touch system typically includes a capacitive sensor panel and a sensor controller. The sensor panel is a multi-layer composite structure composed of a first glass plate having a bottom surface whereon a first conductive layer is formed and a second glass plate having a bottom surface whereon a second conductive layer is formed. The first and second glass plates are typically affixed in a stacked relationship such that the first conductive layer is disposed between the bottom surface of the first plate and the top surface of the second plate. Further, the sensor panel is affixed directly over the screen of a display device (e.g., LCD) such that the second conductive layer is disposed between the bottom surface of the second glass plate and the top surface of the display screen. The conductive layers are typically composed of a transparent conductive material such as, for example, indium tin oxide (ITO) that is deposited by some suitable means (e.g., sputter deposition) and etched in specific patterns. That is, the first conductive layer typically defines a plurality of sensor rows arranged along a y-direction and the second conductive layer typically defines a plurality of sensor columns arranged along an x-direction. Accordingly, the sensor rows and sensor columns, together, define a two-dimensional xy sensor area. The sensor controller is, for example, an microcontroller chip that is electrically coupled to each of the sensor rows and columns so as to monitor their capacitive state. Further, the sensor controller is also electrically coupled to the host device so as to facilitate communication therebetween.
FIG. 1 illustrates the circuitry of a prior art capacitive touch system 100 including a sensor panel 102 and a sensor controller 104. Sensor panel 102 includes a plurality of sensor rows 106ay-fy and a plurality of sensor columns 108ax-fx juxtaposed along the y and x directions, respectively. Each of sensor rows 106ay-fy includes a discrete sensor element 110 and each of sensor columns 108ax-fx includes a discrete sensor element 112. Each sensor element 110 and 112 is a thin pattern of ITO that defines a series of connected diamond shapes extending completely across the touch surface of sensor panel 102 in the x and y directions, respectively. Sensor controller 104 includes a first set of channels 114ay-fy and a second set of channels 116ax-fx. Each of channels 114ay-fy is electrically connected to the sensor element 110 of each respective one of sensor rows 106ay-fy via one of a set of signal lines 118 (e.g., conductive traces, wires, etc.). Likewise, each of channels 116ax-fx is electrically connected to the sensor element 112 of each respective one of sensor columns 108ax-fx via one of a set of signal lines 120. During the operation of touch system 100, controller 104 continuously repeats a cycle of sequentially scanning sensor rows 106ay-fy and columns 108ax-fx so as to measure the capacitive states of their respective sensor elements 110 and 112. There are many known methods for measuring the capacitive state of a sensor element such as, for example, charging the element and observing the settling time. The sample measurement is then compared to a stored value indicative of the elements normal capacitive state in the absence of a target object. When a target object approaches a particular area of sensor panel 102, the natural charge of the target object causes the capacitive state of nearby sensor elements 110 and 112 to change. Algorithms then process the capacitive change in nearby sensor elements 110 and 112 to generate y and x coordinates indicative of the touch location. Controller 104 then provides these coordinates to the hosting device where they undergo further processing for mapping the coordinates to the underlying graphical display device.
Although prior art capacitive touch system 100 has advantages over resistive touch systems, several problems still exist. For example, ITO has a relatively high resistance which imposes constraints on the length of sensor elements 110 and 112. Generally, as the series resistance of a capacitive sensor element increases, the touch sensitivity decreases. Because series resistance increases proportionally with the length of an element, sensor elements 110 and 112 have to be relatively short in order to achieve an acceptable degree of sensitivity. Consequently, the design of touch system 100 is not suitable for use in applications employing large display screens. Although the series resistance can be reduced by increasing the area of the diamond pattern, doing so reduces the sensor resolution of a sensor panel. Also, the series resistance can be reduced by using a low surface resistance ITO layer, but doing so reduces the transparency of the of the sensor elements and makes the sensor elements visible.
FIG. 2 illustrates a prior art capacitive touch system 200 that addresses the size and resolution constraints imposed by the high resistance of ITO. System 200 includes a sensor panel 202, a first sensor controller 204, and a second sensor controller 206. Sensor panel 202 includes a plurality of sensor rows 208ay-hy extending in the x-direction. Sensor panel 202 further includes and a plurality of sensor columns 210ax-gx extending in the y-direction. Each of sensor rows 208ay-hy includes a respective discrete sensor element 212 and each of sensor columns 210ax-gx includes two discrete sensor elements 214 and 216. Sensor controller 204 includes a first set of channels 218ay-dy and a second set of channels 220ax-gx. Each of channels 218ay-dy is electrically connected to a respective sensor element 212 of a respective row 208ay-hy via a respective set of signal lines 222. Likewise, each of channels 220 is electrically connected to one of first sensor elements 214 of a respective sensor column 210ax-gx via a respective one of signal lines 224. Sensor controller 206 includes a first set of channels 226ey-hy and a second set of channels 228ax-gx. Each of channels 226ey-hy is electrically connected to a respective sensor element 212 of a respective row 208ey-hy via a respective one of signal lines 230. Likewise, each of channels 228ax-gx is electrically connected to a second sensor element 216 of a respective one of sensor columns 210ax-gx via a respective one of signal lines 232. The operation of system 200 is similar to that of touch system 100 except that system 200 can support larger screen areas because each of sensor columns 210ax-gx includes two sensor elements (i.e. sensor elements 214 and 216) rather a single sensor column extending across the entire y distance of panel 202. Thus, the y-distance of panel 202 can be twice as long as that of panel 102.
Although system 200 can support a larger screen area than touch system 100, there are still problems with the design. For example, the x-distance of panel 202 still has to be relatively short because each of sensor rows 208ay-hy only includes a single sensor element 212 that extends across the entire x-distance. Accordingly, the design of system 200 only relaxes the constraints in the y-length of panel 202 and, therefore, the constraints in the x-direction still exist. As another example, controllers 204 and 206, together, have a relatively high number of channels (i.e., 218ay-dy, 220ax-gx, 226ey-hy, and 228ax-gx) in order to acquire the capacitive states of sensor elements 212, 214, and 216. In other words, system 200 has a high channel-to-sensor element ratio. Of course, as the number of required channels and, therefore, channel connections, increases, the overall reliability of system 200 decreases. Furthermore, the high number of channels also makes the manufacturing and assembly of system 200 expensive because only controllers that support a high number channels can be used.
What is needed, therefore, is a capacitive touch system design that can be used in conjunction with larger display screens. What is also need is a capacitive touch system design that improves touch sensitivity without sacrificing optical clarity and/or reducing sensor resolution. What is also needed is a capacitive touch system that has a lower channel-to-sensor element ratio. What is also needed is a capacitive touch system that has a higher reliability and cost less to manufacture.